126 
THE PLANT WORLD. 
THE CHIMNEY-SHAPED STOMATA OF THE BURRO 
THORN. 
Dr. C. E. Bessey describes in the Bulletin of the Torrey Botanical 
Club , October, 1904,* a peculiar plant—the “ burro thorn ” which 
was collected some years ago by Mrs. Dorothy Bacon in the Salt 
River Valley near Phoenix, Arizona. This shrub at once attracts 
attention because of its complete leaflessness, and the thorny nature 
of its branches. It is said to grow about 10 feet high, and to 
form an impenetrable thicket from the ground up. It grows in 
the desert where it was first found about fifty years ago by Major 
Emory of the U. S. Army on one of his expeditions. 
The plant is locally known as “ burro thorn,” “ sacred thorn,” 
“ crucifixion thorn,” and by the Mexicans as “ crucifixo ” and 
“ corona de Cristo.” Sometimes people in the locality where it 
grows have called it in pseudo-scientific fashion “ Crucifera spi- 
nosa ” under the impression that it is a good botanical name. 
Although it is usually a low, spreading, much branched shrub, 
it attains in the plains south of Maricopa, Arizona, to the size and 
habit of a small tree. 
Seedling plants bear small leaves, but the mature plant is 
leafless, nothing more than the smallest scales remaining as ves¬ 
tiges. The branches have become modified into spreading thorns, 
which are themselves freely branched again. 
“ All parts of the surfaces of the branches and thorns are of a 
pea-green color, and minutely roughish to the touch. Upon 
making a transverse section the epidermis is found to be of un¬ 
usual thickness, being no less than three, four or five layers of 
cells deep. The outer wall is of excessive thickness, and those 
below, while much thinner, are still very thick. The lateral walls, 
also, are greatly thickened, so that in a superficial view the cell- 
cavities are widely separated. Microchemical tests show that the 
outer epidermal cell-walls are strongly cuticularized, while, those 
below are less so, or not all. Below the epidermis lies a thick 
mass of palisade cells (p), abundantly supplied with chlorophyll. 
These cells are closely packed in about three layers, the cells 
* Bull T. B. C., 31: 9. Oct., 1904. 
